The present invention relates generally to metrology and inspection techniques, which are used in a semiconductor manufacturing processes. More specifically, the present invention relates to techniques for acquisition of wafer targets and measuring an alignment error between different layers or different patterns on the same layer of a semiconductor wafer stack.
Target acquisition is one of the most widespread processes during optical inspection and metrology of semiconductor. Every inspection or metrology recipe to test a semiconductor process requires accurate navigation to the target and identification and acquisition of the feature to be inspected. Presently, target acquisition is typically performed via special acquisition patterns that are printed together with layers of the wafer, next to the positions where the actual inspection or metrology operations are to be performed. The images of these acquisition patterns are captured via an imaging tool and an analysis algorithm is used to verify that the captured image contain the acquisition pattern, and calculate the target image coordinates on the true wafer.
The measurement of overlay and alignment error on a wafer is one of the most critical process control techniques used in the manufacturing of integrated circuits and devices. Overlay accuracy generally pertains to the determination of how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it. Alignment error relates to the determination of how accurately a first pattern aligns with respect to a second pattern disposed on the same layer. The terms overlay and alignment are used herein interchangeably. Presently, overlay and alignment measurements are performed via test patterns that are printed together with layers of the wafer. The images of these test patterns are captured via an imaging tool and an analysis algorithm is used to calculate the relative displacement of the patterns from the captured images.
Conventionally, an object is imaged by an optical tool having a light source for directing incident beams towards the object. The beams are reflected and scattered away from the object towards an image sensor, such as a CCD (charge coupled device) camera. Specifically, there are multiple rays coming from the object. The rays then typically pass through a lens and thereafter form an image of the object at a specific plane, referred to as an image plane. The CCD camera then must be placed at this specific image plane location to achieve a focused image of the target with the least amount of blurring or with the most clarity.
Unfortunately, given conventional mechanical movement mechanisms, it is impossible to position the sensor at the absolute optimum focus position for every acquisition operation or overlay measurement. That is, conventional methods will have tolerances that do not allow movement to a precise enough position for achieving maximum focus for every acquisition operation or overlay measurement.
In addition optical aberrations of the imaging system cause a placement error of the image. For overlay targets, these placement errors are different for the scattered light from the first and second layer. The difference in the aberrations induced placement error causes an error in the overlay measurement. In order to minimize this aberration induced overlay error, accurate centering of the overlay target along the optical axis of the optical system is required.
Accordingly, improved target acquisition and overlay imaging mechanisms are needed. Additionally, a target imaging mechanism that provides flexible placement of the imaging sensor (z) would be beneficial. Additionally, an overlay metrology mechanism that provides flexible placement of the overlay target (x,y) would be beneficial.